The present invention comprises an improvement in the production of mercaptans starting from ethylenic compounds, by reaction of the latter with hydrogen sulphide. It relates, in particular, to the photochemical synthesis of mercaptans from olefins and from ethylenic esters in the liquid phase.
In view of the well-known industrial utility of various mercaptans, methods of preparation of these substances have given rise to numerous studies. One process which has led to interesting industrial results is based on the direct action of hydrogen sulphide on ethylenic substances, under the effect of ultraviolet radiation. In carrying out this process, the technique is to introduce into the reaction medium promoter agents, in particular, organic phosphites, as described, for example, in U.S. Pat. No. 3,050,452. The photochemical creation of SH radicals, which should be induced by the addition to H.sub.2 S to the double bond of the ethylenic compound, implies the utilization of ultraviolet radiation of short wavelength, which causes certain disadvantages. In practice, it is necessary to utilize agents and solvents which are particularly pure and transparent. On the other hand, the optical transmission in the reactor is poor because of the high coefficient of molecular extinction of hydrogen sulphide. In addition, the synthesised mercaptan itself absorbs radiation and this causes a considerable deceleration of the reactions for only slightly elevated degrees of progress, with the formation of sulphides by the addition of the mercaptan itself to the double bond of the ethylenic compound.
In order to avoid these disadvantages and carry out industrially the above-indicated process with suitable reaction speeds and yields, it is thus necessary to be able to effect the photochemical addition over a larger wavelength range, namely approaching the visible spectrum or even with visible light.